This invention relates to an optical fiber amplifier having an optical fiber.
In the general optical fiber amplifier, an optical fiber is doped with a rare earth element into a core portion. In this event, an optical signal is excited by an excitation laser having a wavelength bandwidth of 0.98 .mu.m or 1.48 .mu.m. Under this condition, an amplified optical signal of 1.55 .mu.m bandwidth is obtained when an optical signal of 1.55 .mu.m bandwidth is given.
The excitation method includes a forward excitation, a backward excitation and the bidirectional excitation. In a forward excitation, an excitation light beam is given to an input side of the optical fiber to perform the excitation in a forward direction. In the backward excitation, the excitation light beam is given to an output side of the optical fiber to perform the excitation in a backward direction. In the bidirectional excitation, excitation light beams are given to the input side and the output side to perform the excitation in both the forward and backward directions.
Generally, the forward excitation serves to reduce a noise index while the backward excitation serves to obtain a high output power. The bidirectional excitation serves to reduce the noise index and to obtain the high output power. In the practical optical fiber amplifier, the bidirectional excitation is often used to optimize both the noise index characteristic and the output power characteristic.
The conventional amplifier for the bidirectional excitation has an input side wavelength combiner and an output side wavelength combiner. The doped optical fiber is connected between the input side wavelength combiner and the output side wavelength combiner. In this event, a first excitation laser device is connected to the input side wavelength combiner while a second excitation laser device is connected to the output side wavelength combiner. With such a structure, an input optical signal is excited by an excitation laser light beam from the first excitation laser device in the input side wavelength combiner to supply the excited optical signal to the doped optical fiber. On the other hand, the optical signal from the doped optical fiber is excited by an excitation laser light beam from the second excitation laser device in the output side wavelength combiner to supply the excited optical signal as the output optical signal. Thus, the bidirectional excitation is carried out in the conventional amplifier.
However, cost and consumption of power are largely increased in the conventional amplifier because the conventional amplifier has two laser devices in the forward and backward directions.